Tag i18n strings in templates

First pass at tagging translation strings in the templates.  The main goal is to do extraction for proofing the text, so this isn't 100% exhaustive.  We're still missing some of the "x of y" strings (which are hard to translate out of context) and all of the JavaScript (which will require some changes to XBlock SDK before we can assume that `gettext` is loaded on the page).

@stephensanchez